Electron beam control means



Oct. 30, 195

Filed Jan. 21, 1954 M. J. OBERT ELECTRON BEAM CONTROL MEANS 2Sheets-Sheet l IN VEN TOR.

TORNEY Oct. 30, 1956 OBERT ELECTRON BEAM CONTROL MEANS Filed Jan. 21,1954 2 Sheets-Sheet 2 LVI'ENTOR.

TOR NE 1 United States Patent 7 2,769,110 ELECTRON BEAM CONTROL MEANSMaximilian J. Obert, Camden, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application January 21, 1954, SerialNo. 405,445 4 Claims. (Cl. 31370) This invention is directed to acathode ray tube and more specifically to a cathode ray tube having aplurality of electron beams which are converged to a common point on atarget electrode.

One type of cathode ray tube which uses a plurality of electron beams isthe television picture tube for color of the type disclosed in U. S.Patent 2,595,548, Alfred C. Schroeder. It has been found advantageous intubes of this type to utilize an electron gun assembly of the typedisclosed in the copending application of Albert M. Morrell, SerialNumber 383,340, filed September 30, 1953. The tube described in thiscopending application has three electron guns mounted in the neck of thetube envelope symmetrically about the tube axis and inclined to the tubeaxis by a small angle to provide three beams converging to a commonpoint on a target electrode mounted normal to the tube axis.

The target electrode, of the Morrell application device; cited above,includes a masking apertured electrode formed of sheet metal such as acopper-nickel alloy and having a large number of small aperturestherethrough. Closely spaced from the surface of the masking electrode,opposite the electron guns is a glass sheet having on its adjacentsurface a large number of phosphor dots in groups of three, with eachdot of each group fluorescing when struck with high energy electronswith a diiierent color of light than the other dots of the group. Eachgroup of three dots is positioned relative to one aperture of themasking electrode so that the center about rhich the three dots of eachgroup is positioned is aligned with the respective aperture of the mesh.Electrons from the three beams, passing through any one of the aperturesin the masking electrode, approach the corresponding group of phosphordots from their three different directions. In this manner, theelectrons passing through an aperture of the masking electrode along onedirection will strike only one phosphor dot and will be prevented by themasking electrode from striking the other two dots of the group; Thearrangement provides, then, that an electron beamfromteach gun of thetube will cause the phosphor screentoluminesce in only one color.

Accurate'convergence of the three electron beams at the" target dependsupon precisealignment of each gun and gun parts. 'Misalignment of] theguns or gun parts results in displacementof'one or'more of the beamsfrom thepoint of common convergence. The device shown in the copendingapplication of Albert M. Morrell uses independent magnetic fields forcorrecting for any misconvergence of the beamson the target. Independentconvergence control for each beam is provided by a plurality of pairs ofpole pieces extending transversely of and on opposite sides of each beampath. Portions of the pole pieces are positioned adjacent the envelopeneck of the tube whereby an electromagnet mounted on the tube neckadjacent each pair of pole pieces can be used to establish a convergencecorrecting magnetic field across each beam path. 7 An additional pair ofmagnetic field forming pole pieces is mounted adjacent one of the threeguns to provide a second correcting magnetic field at right angles tothe other field of that gun to complete the convergence control of thebeams, to be described.

The tube of the above cited application to Morrell uses an externalelectromagnet with appropriate armature portions for providing thesecond correcting magnetic field to provide convergence of all threebeams at the target.

It is an object of this invention to provide permanent magnetic meansfor providing the second correcting magnetic field of the tubedescribed.

it is another object of this invention to provide an adjustablepermanent magnetic means to form a convergence correcting magnetic fieldfor a tube of the type described.

it is a further object of the invention to provide a simplified low costpermanent magnetic means for forming the convergence correcting field ofthe tube of the type described.

The invention is in a low cost simplified adjustable permanent magnetfor forming a convergence correcting field in a cathode ray tubeutilizing a plurality of electron beams for color televisionreproduction. A threaded magnet is mounted exteriorly of the cathode raytube neck over the electron gun which furnishes the blue component ofthe television picture. The threaded magnet is mounted in a holdercomprising a permeable flux conductor member, the ends of which rest onthe tube neck adjacent to internal portions of a pole piece element. Thedesign and arrangement of the magnet enables the forming of a magneticfield which may be varied or adjusted to correct for misconvergence ofthe blue beam at the target. Furthermore, the magnet is arranged toprovide a field adjustment passing through zero to both sides so thatbeam deflection of the blue gun can be in either direction.

Figure l is a sectional view of a ing the invention.

Figure 2 is an enlarged perspective view of a portion of the target ofthe tube in Figure 1.

Figure 3 is an enlarged sectional view of part of the gun structure ofthe tube of Figure l.

Figure 4 is a cross sectional view along line 44 of Figure 3.

Figure 5 is a cross sectional view along line 55 of Figure 3.

Figure 6 is a schematic representation of beam convergence at the targetelectrode of the tube of Figures 1-5.

Figure 7 is a view of a modification of the invention.

Figure 1 represents a cathode ray tube of the type disclosed in thecopending application of Morrell. The tube has a plurality of electronbeams and is used as a picture viewing tube for color television. Thetube consists of an evacuated envelope having a neck portion 10 andalarge metal shell portion 12. Within theneck portion if; of the envelopeare three electron guns 13, each consisting of a cathode electrode 14(Figure 3), a control grid electrode 16 and an accelerating electrode25). Spaced along the axis of each gun from accelerating electrode 20 isa relatively long tubular accelerating electrode 22,'and a shortfocusing electrode 39. The electrodes 39 are mounted on a common supportplate 28. Mounted on the opposite face of plate 28 from electrodes 3%and between it and another support plate 26 are a plurality of polepieces 53. The pole pieces 58 are arranged in pairs, with one pairpositioned on oppositesides of each beam as shown in Figure 4.

The electrode assembly of plates 25 and 28 is supported from andelectrically connected by spring finger-s or spacers 34 to the wall ofthe envelope neck 10 to make cathode ray tube utilizelectrical andphysical contact with a conductive wall coating 36, which extends overthe inner surface of the tubular envelope neck portion from plate 28into the conical envelope portion 12 to make contact with the metalshell 12.

The several electrodes are mounted within the neck portion 10 by rigidlyfastening them together by means of a plurality of glass mounting rods38. Lead wires 42 extend from each electrode respectively, and areconnected to base pins 45 extending through base 43'to make electricalcontact to external sources of potential. Lead wires 42 and the springspacing fingers 34, rigidly support the gun assembly within the envelopeneck portion 10.

In Figure 3, voltages are indicated as those which are applied to therespective gun electrodes. These voltages are those which have beensuccessfully used in tubes of the type described and need not belimiting.

During the operation of the tube described, potentials are applied toseveral gun electrodes in the amounts indicated. The electron emissionfrom each cathode 14 is formed by electrostatic fields respectivelybetween electrodes 16, 20, and 22 into an electron beam directed throughthe apertured portions of the gun electrodes; The difference ofpotential between electrodes 22 and the corresponding electrode portions30 provide a principal focusing lens field in the path of each electronbeam, whereby the electrons of each beam are converged to a fine focusat the target electrode 44 mounted in the large shell portion 12 of theenvelope.

Target electrode 44 consists of a glass support plate 46 and a metallicmasking electrode 48 closely spaced from the surface of plate 46 facingthe electron guns. Masking electrode 48 is a thin copper-nickel sheethaving a large number of small apertures 50. Fixed to the adjacentsurface of the glass plate 46 is a luminescent screen consisting ofgroups of phosphor dots 52, with each group consisting of three dotspositioned in a triangular arrangement about a center point 54, as shownin Figure 2. The positioning of each group of phosphor dots is such thatthe center of each aperture 50, in the masking electrode 48, will bealigned with the point 54 of the corresponding group of phosphor dots.

The phosphor dots 52 of each group are formed of phosphor materialfluorescing with a different colored light when struck by the highenergy electrons from guns 13. .As indicated in Figure 2, the dots ofeach group have a red, green, or blue fluorescence under electronbombardment and indicated respectively by R, G, and B. Furthermore, thepositioning of the phosphor dots 52 is that in which each dot is alignedwith its corresponding aperture 50 in electrode 48 along a differentdirectional line X, Y, and Z, respectively. The fluorescent screen iscovered with a thin film 53 of reflective metal to intensify theluminescence of the phosphor by reflecting light from the phosphorscreen through plate 46 toward the observer.

The three'electron beams leaving the electron guns 13 are caused toconverge toward the common axis 17 of the tubular 'e'nvelope portion 10by mounting each gun 13 at a small angle to axis 17 so that the axes ofthe three guns will converge to a common point on the masking electrode48. Thus, each beam, normally following the axis of its gun, willapproach the masking electrode 43 at a small angle of incidence and fromone of the different directions X, Y, or Z. Electrons from each beampassing through the apertures 50 of electrode 48 along one of the pathsextending in the directions X, Y, or Z, will strike one phosphor dot ineach group of dots. The arrangement is such that the electrons from eachgun can strikeonly those phosphor dots 52 luminescing with a singlecolor of light. The angle which each gun makes with tube axis 17 issmall and is determined by the dimensions of the tube. In tubes of thetype described which have been successfully operated, this angle is inthe order of 1 10'. The Figure 3 exaggerates the angle between the gunsand axis 17 for purpose of illustration.

The three beams are simultaneously scanned over the surface of themasking electrode 48 by conventional scanning means indicated as a neckyoke 56, which consists of two pairs of deflecting coils, with the coilsof each pair mounted on opposite sides of the envelope neck 10. Eachpair of deflecting coils of yoke 56 is connected in series to sources ofsaw tooth currents for providing line and frame scansion of the threeelectron beams simultaneously over the surface of the masking electrode48 The scanning coils of yoke 56 are conventional'and do not constitutea part of this invention and need not be further described. The scansionof electron beams may be in any desired manner but for color televisionviewing is as a rectangular raster. The operation of tubes of the typedescribed are more fully set forth in copending application SerialNumber 231,925, filed June 16, 1951, Pat. No. 2,663,821, by Harold B.Law.

Due to any misalignment of the gun parts as well as non-uniformity ofany field affecting the beams, the three beams may not normally coincideaccurately at the same point. Figure 6 indicates that if the three beamsare accurately positioned symmetrically about axis 17, and with nodeflection fields acting on the beams, they will strike a single spot 0on axis 17. However, due to misalignment of gun parts or non-uniformityof fields through which the beams pass or both, one or more of the spotsR, B, and G may be displaced from the spot 0, which they all shouldtheoretically strike and will strike in new spots R, G, and B, forexample.

Beam converging means are provided consisting of the pairs of polepieces 58 mounted on opposite sides of the electron beam paths. As shownspecifically in Figure 4, the pole pieces 58 include parallel portionsextending substantially radially to the axis 17 of the envelope neck 10.Also, the pole pieces 58 each have an arcuate portion 60 extending alongthe inner wall portions of the tubular neck 10 and as shown in Figure 4.The arcuate.

pole pieces portions 60 are matched with arcuate portions 62 ofarmatures64 forming respectively a part of the electromagnets 66 mountedon the outer wall portion of the tubular neck 10.

A direct current is passed through each coil 66 to provide a constantmagnetic field between each pair of plates 58. By adjusting the strengthand direction of the direct current in each coil 66, each electron beamis moved radially toward tube axis 17. This will move the three spots R,B, and G together along the lines 61, 63 and respectively, and in thedirection indicated by the arrows,

shown in Figure 6. However, the three beams can not now be converged toa single common point, although the red and green beam spots, can beconverged to a common spot 0'. v

Accordingly, then a correction is introduced into the deflection of oneof the beams by a magnetic field established between pole pieces 70 and72, shown in Figures 3 and 5. Pole pieces 70 and 72 are fixed to thetubular electrode 22 of the blue gun 13 and are mounted at substantiallyright angles to the corresponding plates 58 of the gun. A magnetic fieldbetween the parallel pole pieces 70 and 72 will introduce a beamdeflection at right angles to that produced by plates 58 on the samebeam. As shown in Figure 6, this shifts the convergence path 63 of beamB to a new position 63', and thus allows beam B to be converged alongpath 63' to the intersection of spots R and G at the point 0. The point0' is not greatly displaced from the theoretical point 0. Thedisplacement can be compensated for by other means if necessary.Additional correcting plates (not shown) similarly to 70 and 72, foreach beam, may be used if necessary or desirable.

The correcting magnetic field established between parallel plates 70 and72 may beformed by extending pole piece 70 adjacent to the neck portionof the tube envelope to form flanged end portions 73. Pole piece 72 is abox-like construction and the wall of the box opposite gun electrode 22is arcuate to follow the curvature of the tube neck 10.

In accordance with this invention, the correcting magnetic field for theblue gun 13 is established between plates 70 and 72 by providing apermanent magnet means exteriorly of the tubefenvelope. As shown inFigure 5, the magnet means consists of a permanent magnet rod 76 mountedin the central portion of an arcuately formed flux conductor member 78.The magnet 76 may be threaded through a supporting stud or positioningblock 77 mounted on the central portion of the flux conductor 78 so thatthe magnet extends radially of the tube axis 17. One end of the magnet76, as shown in Figure is positioned over pole piece 72. The fluxconductor 78 has a pair of end portions 74 which are bent in the mannershown in Figure 5 to overlie the flanged end portions 73 of pole piece70. Pole piece 72 is of a soft magnetic material such as a nickel-ironalloy having high permeability. Also pole piece 70 may be of the samematerial. The flux conductor 78 may be made of a soft magnetic materialsuch as low carbon steel and is used to support the magnetized rod 76. Anonmagnetic spring 75 is attached to both of the end portions of theflux conductor 78 and extends around the tube neck to hold the assemblyin position during operation.

The magnet rod 76 is magnetized axially and is long enough so that eachend may be considered a point source of magnetic fiux. The fluxconductor 78 shapes and directs the magnetic flux of magnet 76 and actsas a shield when the magnet 76 is in retracted position. With magnet rod76 in position, as shown, the magnetic flux will extend from themagnetized rod through both legs 74 of the flux conductor through theglass wall of the tube and into the adjacent flanged ends 73 of polepiece 70, through the tubular electrode structure to the pole piece 72and from this pole piece through the glass wall of the tube back to theend of the magnet rod. The distance of the end of the magnet rod frompole piece 72 can be adjusted by the threaded construction of the rod.

The insertion of the magnet rod into the threaded assembly 77 causes themagnetic flux lines in the electrode of the blue gun to move theelectron beam of this gun until convergence is achieved, as is describedabove relative to Figure 6. By reversing the magnet, adjustment oneither side of the uncorrected blue beam position can be achieved asrequired by the characteristics of the tube used. Upon insertion of theend of the magnet into the support assembly, the blue beam is firstdisplaced a small amount in one direction which is opposite to thedisplacement of the beam when the magnet is positioned farther into theassembly. The red and green beams are also displaced slightly laterally,at the first insertion of the magnet into the assembly. However, afterthis first displacement, the red and green beams are not displaced uponmoving of the magnet through its threaded support toward the tube neck,since the red and green beams are shielded from the flux of the magnetrod by the plate 70 and their own gun structures. The fact that the blueelectron beam can be moved in both directions depending upon the amountof insertion of the magnet rod into the assembly permits the adjustmentof the electron beam through a zero position and as well as adjustmentof the beam to zero position. The reasons that the electron beam of theblue gun is first deflected in one direction when the magnet rod 76 isfirst inserted position of the magnet, the flux lines between the endsof the rod 76 pass through the open ends of the arcuate flux conductor78, and in the region of the blue gun have a weak component in thedownward direction. However, as the south pole end of the magnet rod isinserted 6 through the support assembly 77, flux between the pole' piece70 and the adjacent south end of the magnet rod increases in strength toneutralize the downward component of the flux and finally results in astrong upward which continues to increase as said flux conductor, saidcorrecting field.

3. An electron discharge device comprising an envelope having a tubularportion, an electron gun mounted within said tubular envelope portion,said gun including a plurality of electrodes mount-ed along a commonaxis tor forming a beam of electrons, a pair of pole pieces mounted onone of said electrodes, for forming a magnetic correctingfield in thepath of said electron beam, said pole pieces including a flat plateextending across said tubular envelope portion and having flanged edgeportions positioned adjacent the Wall of said tubular envelope portionand a tubular pole piece member fixed to the opposite side of said oneelectrode from said fiat plate, said tubular pole piece having anarcuate Wall extending adjacent to the wall of said tubular envelopeportion, and an armature mounted on said tubular envelope portion, saidarmature including a median portion extending over said arcuate Wall ofsaid tubular pole piece and end portions extending to the Wall of saidtubular envelope portion, each of said armature end portions beingflanged and overlying one of said flanged edge portions of said flatplate pole piece, and a magnetized rod adjustably mounted through saidmedian armature portion.

4. An electron discharge device comprising an envelope having a tubularportion, a target electrode structure mounted within said envelope, aplurality of electron gun structures Within said tubular portion spacedfrom said target electrode, each of said gun structures including aplurality of electrodes mounted along a common axis for 8 forming a beamof electrons, said gun structures being arranged so that said respectivecommon axes converge to' a point at said target electrode, a pair ofmagneticpole pieces mounted adjacent one of said guns for forming amagnetic correcting field in the path of the respective electron beam,said pole pieces including portions p ositioned adjacent the wall ofsaid tubular envelope portion, magnetizing means mounted on said tubularenvelope portion including a flux conductor'having end portions adjacentthe wall of said tubular envelope portion and overlying said adjacentportions of-said pole pieces, and a permanent magnet means mountedonsaid flux conductor, said -magnet means including structuge foradjustably varying said correcting field.

Refer'cnces Cited in the file of this patent UNITED STATES PATENTS2,188,579 Schlesinger Jan. 30, 1940 2,200,039 Nicoll May 7, 19402,455,676 Hillier Dec. 7, 1948 2,500,455 Fisher Mar. 14, 1950 2,513,221Webb June 27, 1950 2,542,924 Heppner Feb. 20, 1951 2,574,039 Ingle etal. Nov. 6, 1951 2,586,948 Heppner Feb. 26, 1952 2,642,546 Patla a June16, 1953

